A high-axle load railway such as a mining railway mainly carrying mineral ore is large in carrying capacity of a train or a freight car. In such a railway, a load applied to an axle of a freight car is extremely large compared with a passenger car, in addition, use environment of a rail is more severe. For a rail used in such an environment, steel having a pearlitic structure has been mainly used from a point of significant concern of wear resistance. However, recently, carrying capacity of a freight car is further increased for efficient railway transportation, so that use environment of a rail becomes more severe, and consequently further improvement in wear resistance or rolling contact fatigue (RCF) resistance is required for the rail.
To meet such requirement, from the point of significant concern of wear resistance or RCF resistance, a rail is aimed to be increased in strength, and a high-strength pearlitic steel rail having a tensile strength of 120 kg/mm2 (1200 MPa) or more is proposed as shown in Japanese Unexamined Patent Application Publication JP-A-7-18326. However, it is known that possibility of delayed fracture is increased in high-strength steel having a tensile strength of 1200 MPa or more. While high strength is obtained by the technique shown in the JP-A-7-18326, adequate delayed fracture properties are not obtained by the technique.
As a technique for improving delayed fracture properties of high-strength pearlitic steel, for example, Japanese Patent No. 3,648,192 and JP-A-5-287450 disclose a technique that high-strength pearlitic steel is subjected to high wire drawing process so as to improve delayed fracture properties. However, when the technique is applied to the rail, a problem occurs, that is, the high wire drawing process causes increase in manufacturing cost.
As a method of improving delayed fracture properties other than the above, it is known that a figure and volume of A type inclusions are effectively controlled. JP-A-2000-328190, JP-A-6-279928, Japanese Patent No. 3,323,272, and JP-A-6-279929 disclose such control of the figure and volume of A type inclusions in rail steel respectively. However, each of JP-A-2000-328190, JP-A-6-279928, Japanese Patent No. 3,323,272, and JP-A-6-279929 aims to improve toughness and ductility of a rail, and does not always provide excellent delayed fracture properties. For example, JP-A-6-279928 discloses a method where size of an A type inclusion is controlled to be 0.1 to 20 μm, and the number of A type inclusions is controlled to be 25 to 11,000 per square millimeters, so that toughness and ductility of a rail are improved. However, excellent delayed fracture properties are not always given by the method.
On the other hand, Japanese Patent No. 3,513,427 or Japanese Patent No. 3,631,712 discloses that Ca is added for improving toughness and ductility of a material for a rail. For example, Japanese Patent No. 3,513,427 discloses a method where Ca of 0.0010 to 0.0150% is added to produce a sulfide in a form of CaS, and the CaS is used to finely disperse MnS, so that a Mn dilute zone is formed around MnS so as to contribute to occurrence of pearlite transformation, and block size of such pearlite is refined, thereby toughness and ductility of a rail are improved.
However, while the methods are useful to improve toughness and ductility, they do not take delayed fracture properties into consideration. Moreover, when the added amount of Ca is increased, since rough and large C-type inclusions are generated in steel, RCF resistance is reduced. Here, the A type inclusion and the C type inclusion are those defined in Appendix 1 of JIS (Japanese Industrial Standards) G0555.